This proposal outlines a two-year transitional grant for Dr. Jason L. Kubinak, Ph.D. Award guidelines stipulate that this grant becomes active once Dr. Kubinak begins his appointment as an assistant professor at a research institution, and that the funds will be used to support his first two years of work. Dr. Kubinak is currently a postdoctoral research fellow in the lab of Dr. June L. Round, Ph.D. at the University of Utah. Dr. Kubinak's research focuses on understanding the immune mechanisms controlling antibody responses against commensal microbes in the gut and how altered host-microbiota relationships contribute to disease. Dr. Kubinak has recently demonstrated that disruptions to Toll-like receptor (TLR) signaling in T cells alters antibody-mediated selection in the gut that results in establishment of a more pro- inflammatory microbiota. He is now focused on understanding whether polymorphism at major histocompatibility complex (MHC) genes, can promote a microbial self that explain patterns of disease susceptibility among individuals. Along with classic immunological methods, Dr. Kubinak has been employing next-generation sequencing approaches to characterize microbiota communities among individuals as well as the effect of host MHC genotype on patterns of selection on developing immunoglobulin (Ig) repertoires in the gut. His data indicates that loss of MHC antigen presentation, and MHC polymorphisms, are associated with dramatic shifts in gut microbiota composition and that this is associated with multiple phenotypic differences in antibody responses against commensals. More importantly, he also has data indicating that MHC-mediated patterns of susceptibility to enteric infection are dependent on how MHC shapes unique communities among individuals. For this proposal Dr. Kubinak is seeking to build off of his postdoctoral work to understand how TLR and MHCII signaling converges in B cells to direct antibody responses against commensals. These two pathways skew between T cell-independent (TI) B cell responses generating polyclonal low-affinity antibodies, and T cell-dependent (TD) B cell responses generating monoclonal high-affinity antibodies. To contrast the importance of these two pathways on antibody responses against commensals in vivo, two B cell conditional knockout mice will be developed that have TLR signaling (MYD88fl/flCD19cre+) or MHCII presentation (MHCIIfl/flCD19cre+) knocked out. This will produce two mouse models where B cells mature in response to TLR ligands (modelling a TI- skewed phenotype) or MHC-presented peptides (modelling a TD-skewed phenotype), respectively. These models will be used to contrast how TI- versus TD-biased responses influence IgA repertoire selection and IgA-mediated targeting of commensal species, how this influences microbiota composition, and how altered microbiota communities differentially influence susceptibility to inflammatory and infectious enteric diseases. Finally, D. Kubinak will also address whether exposure to a ubiquitous environmental pollutant (Bisphenol A) can influence disrupt normal IgA repertoire selection to promote a disease-causing microbiota. The work outlined in this proposal seeks to elucidate the mechanism of natural selection on developing Ig repertoires in the gut and the relevance of the microbiota to this process. This work represents a novel application of evolutionary theory to understanding host health that could yield novel therapies for the treatment of human disease. Dr. Kubinak has spent his doctoral and postdoctoral career focused on understanding the factors governing host-microbe symbiosis. The professional environment provided to him during his postdoctoral training has been instrumental in driving him to understand how evolutionary theory can help us better understand why disease happens. Beyond creating a vibrant and successful research laboratory in the short-term with the experiments described above, Dr. Kubinak's long-term career goal is to enhance the use of evolutionary theory in medicine. To do this, a major component of my career development plan will be to form productive collaborative relationships with basic scientists as well as clinicians, and to promote educational opportunities for students to gain experience in the practical application of evolutionary theory to the understanding and treatment of human disease.